Internal monologue, also known as interior monologue, inner voice, internal speech, or stream of consciousness is thinking in words. It also refers to the semi-constant internal monologue one has with oneself at a conscious or semi-conscious level.

Much of what people consciously report "thinking about" may be thought of as an internal monologue, a conversation with oneself. Some of this can be considered as speech rehearsal, and it seems to be that the internal monologue is generally in the native language of the person concerned.

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In fiction, when one person reads the mind of another, it is often described as being able to hear this internal monologue as if it were said out loud.

When children are taught to read out loud and then later taught to read quietly, they often subvocalize. This has led to a discipline called Speed reading that attempts to suppress this.

In some conditions there is an uncertainty about what the source of these internal sentences is. Attribution for a recently produced internal sentence may lead to concerns over schizophrenia, hallucinations, or hearing voices.

The religious practice of Zen attempts to quiet the internal voice by various means.

Inner speech is by nature a person's authentic set of expressions, revealed before internal censors can come into play, truly free speech.

In order to create internal speech, many of the various components of normal language production must come into play. The mental faculties that deal with semantics and syntax do just as much work in creating an internal sentence as they do in creating one for external use.

Integral in the composition of sentences is the ability to perform word choice and naming. If a
concept is known (e.g. furry pet that meows), the actual word to be used still has
to be decided upon. This involves being language specific (chat vs gato vs cat).

Over a period of time the topic of a stream of conscious will wander. How the stream changes can be described as the flow. Various internal and external causes may affect the flow, including smells, music, and memories. Changes in the flow of consciousness can be logical or seemingly quite random. In the latter case, the subconscious may be at work.

If a sentence is assembled every couple of seconds, then critical events associated with
the production and consumption of the subunits of the internal monologue should be on a
10 or 100 millisecond timescale. On the other hand, with the ability to place certain
topics on hold and reassess them later, the timescale associated with thought trains could
be as long as years.